Electronic and electrical devices are often mounted on a substrate. The requirements of such substrates include being an electrical insulator and in certain circumstances being a heat conductor. Typically, substrates have been used in the manufacture of circuit boards. Circuit boards generally form part of low power electronic devices. Even low power electronic devices generate significant heat, which must be conducted away from the heat generating device.
One substrate that has been identified as providing the properties of being an electrical insulator and a heat conductor is anodised aluminium or anodised aluminium oxide.
GB 2162694 describes a printed circuit board using anodised aluminium as the substrate on which is mounted components, such as a power transformer, of a circuit. The substrate includes heat sinks and a power transformer is mounted directly on the heat sink.
GB939394 describes an electric circuit structure which comprises a supporting member of conductive material and a non-conductive metal oxide coating of the same metal provided by anodising aluminium.
U.S. Pat. No. 4,015,987 describes a chip carrier and process for making the same. The substrate is anodized aluminium the top surface of which is coated with an adhesive and a thin copper foil laminated thereto. A photo resist is applied to the copper foil, followed by etching or plating.
U.S. Pat. No. 5,687,062 describes a thermally efficient circuit board which comprises a metal base such as aluminium, an anodized layer which may be of the same metal as the base, and a thin layer of insulative sealant.
Whilst anodised metal substrates do have improved thermal conduction and voltage breakdown characteristics when compared to resin based substrates often used in circuit board construction, the characteristics of anodised metals do limit the applications where they can be used. An anodised aluminium substrate cannot meet the requirements for use with high power devices, i.e. devices typically dissipating 20 watts at operating voltages in the range 12 volts dc to 300 volts dc and currents in the range 1 amp to 150 amps peak, with a break down voltage from the circuitry to ground of greater than 2.5 kV ac and 3.5 kV dc for safety reasons. The anodised aluminium substrate would break down because it does not have a sufficiently great voltage withstand and cannot dissipate the heat generated by the high power electronic devices.
The anodised aluminium substrate described in WO 96/33863 seeks to address the problem of poor voltage breakdown performance of anodised aluminium substrates. This is achieved by controlling the size of intermetallics within, and the surface roughness of the anodizable metal core. A dielectric polymer coating may also be applied to the anodized surface.
As mentioned above, the presently used substrates have been used in the production of printed circuits, but such substrates do not have characteristics which would allow them to be deployed for certain applications of electronic equipment. It would therefore be desirable to provide a substrate which can be deployed across a wider range of applications of electronic equipment.
WO 03083181 describes a process commonly known as plasma electrolytic oxidation which is a process developed for the treatment of light alloys. In the process a pulsed voltage is passed through a bath of electrolyte solution and applied to the substrate. The resulting plasma discharge rapidly produces a hard, fused ceramic layer on the surface of the component. The resulting ceramic coating consists of two distinct layers; a fused crystalline layer closest to the substrate which is extremely hard and dense; and a thinner porous outer layer. The resulting coating can withstand very high temperatures and is resistant to cracking. The coating is also able to withstand much larger thermal shocks than is the case for hard anodized aluminium. The resulting substrate may be modified to have certain desirable technical characteristics such as resistance to heat, or acting as a dielectric. These modifications are achieved by the addition of ultra-disperse insoluble powders (nano-powders) to the electrolyte during formation of the coating. The material is used in many applications where hard surfaces are required in areas exposed to extreme heat and have been identified as being suitable for use in the mounting of electrical components and to coat materials used to carry electronic windings and circuitry.
U.S. Pat. No. 6,197,178 describes an alternative method of plasma electrolytic oxidation. The process of plasma electrolytic oxidation may be referred to as electrochemical micro-arc oxidation is or micro-plasmic oxidation.
Substrates having a surface coating created by plasma electrolytic oxidation are not known for use in the manufacture of printed circuit boards. The present invention utilises the properties created by plasma electrolytic oxidation to provide an improved printed circuit board.
The invention also provides for the interconnection of high power devices.